DAKAR (Thomson Reuters Foundation) - Tens of thousands of children in northeast Nigeria will die of malnutrition this year unless they receive treatment soon, the United Nations said on Friday after reaching areas of the country previously cut off from aid by Boko Haram violence.

Over the last year Nigeria's army, aided by troops from neighboring countries, recaptured most of the territory that was lost to the militant group, which has waged a seven-year insurgency aimed at creating an Islamic state in the northeast.

"Improving security has enabled humanitarians to access areas that were previously cut off," Munir Safieldin, the U.N. humanitarian coordinator for Nigeria, said in a statement.

The conflict, which has killed more than 15,000 people and uprooted 2.4 million in Nigeria, Chad, Niger and Cameroon, has also pushed food insecurity and malnutrition to emergency levels in northeast Nigeria, according to the Nigerian government.

More than a half a million people need urgent food aid, as the violence has hit farming, disrupted markets and driven up food prices, several U.N. agencies said in a joint statement.

Almost 250,000 children under the age of five in Borno state will suffer from malnutrition this year, said Jean Gough, Nigeria representative for the U.N. children's agency UNICEF.

"Unless we reach these children with treatment, one in five of them will die," she said. "We cannot allow that to happen."

While the United Nations and its partners have gained access to several areas in Borno in recent months, it said many remain unreachable due to the ongoing violence and lack of security.

The medical charity Medecins Sans Frontieres (MSF) last week gained access to a camp in Borno's city of Bama, hosting 24,000 people, including 15,000 children, for the first time since it was seized back from Boko Haram in March 2015.

More than 1,200 people have died from starvation and illness in the camp on a hospital compound, according to MSF, who said "a catastrophic humanitarian emergency" was unfolding in Bama.

Nigeria's army last week said it had freed more than 5,000 people held by the Islamist militant group Boko Haram during an operation in the northeast of the country.

However, the jihadist group, which last year pledged loyalty to Islamic State, still regularly stages suicide bombings, mainly in crowded areas such as markets and places of worship.